The electric guitar and its companion amplifier were developed during the era of vacuum-tube amplification, and consequently, guitar pickups and then-current amplifier designs were empirically optimized to work well together. In particular, guitar pickups were wound with many turns of fine wire, in order to produce a high signal voltage that would minimize the number of vacuum tube amplifier stages required. Such a coil has a high inductance, and therefore requires a high input impedance to avoid high frequency losses. Vacuum tubes have naturally high input impedance, and therefore perform well with such pickups. The output impedance of simple vacuum tube amplifiers is also naturally high, which helps overcome the effects of series inductance in the loudspeaker coil, especially in open loop designs, which were often used to reduce cost. This increases the effective bandwidth reproduced by the speaker. The combined results of both effects produced a desirably warm and lively sound with a natural enhancement of harmonics, despite the lack of sophisticated tone controls in early amplifiers.
Early amplifier designers could not be certain how much voltage would be produced by the various types of guitars available, and each tube stage adds a large increment of gain. Therefore it was common for early amplifiers to have considerably more gain than required for undistorted operation. It was empirically discovered by guitar players that this “excess gain” could be used to add extra loudness, sustain, and harmonics to their sound by intentionally driving their amplifier into clipping. The exploitation of such “overdrive” has become a large part of the tonal range associated with the electric guitar, especially when used in high-energy music.
Although certain details of tube-circuit design were empirically optimized for the playing of electric guitar, it was largely fortuitous that amplifier tubes developed for general purpose audio reproduction proved to have musically valuable properties in guitar amps. As a result, certain dynamic behaviors typical of tube amps, and incorporated into the tradition of electric guitar performance, have been taken for granted, and are not well recognized by amplifier designers. When the industry began a shift to solid-state amplifiers for well-known reasons of cost, efficiency, weight reduction, and component lifetime, more exacting musicians were not satisfied by the resulting performance, because dynamic behaviors of tube amplifiers were missing. Therefore, discerning musicians continue to prefer tube amplifiers as having better tone and dynamic “feel”, despite their well-known drawbacks of high cost, weight, fragility, and increasingly hard-to-obtain vacuum tubes.
Solid-state amplifiers suffer from some first-order drawbacks which have been recognized and partially corrected. One obvious problem is that typical bipolar-type transistors are inherently low-impedance devices which do not mate well with high-impedance pickups. The development of low-power FET input devices, and particularly, FET-based operational amplifiers, can provide the high impedance inputs for solid-state amplifiers that allow the guitar pickup to develop its full frequency range. It is now also common to provide means for controllable overdrive clipping, sometimes with a soft clipping knee, and thus, allowing the musician to transition from the undistorted or “clean” range into overdrive as a function of their volume settings and playing attack. Even with controllable overdrive clipping, solid-state-amplifiers are still considered cold and brittle. In response, various controls have been added to conventional amplifiers that expand the possibilities of sound shaping and add special effects. For example, some devices have added audio transformers (as required in tube amplifiers), or a small-signal tube in the pre-amplifier section of the solid-state amplifier in the belief the additions would provide the missing warmth and tonal qualities of conventional tube-amplifiers. However, even with these undesirable complications, solid-state amps are still considered to be relatively cold, unresponsive, and forced-sounding, in contrast to the warmth, liveliness, and natural “feel” of well-designed tube amps.
More recently, a major effort has been made to use the power of digital signal processing (DSP) to reproduce the dynamics and tonal qualities of tube-amplifiers missing from standard solid-state amplifiers. The industry provides “modeling amps” that purport to reproduce the sounds of various “tone standards” typically associated with well-regarded tube amplifiers. It is generally observed that such schemes are moderately successful for certain defined playing styles and loudness levels, but still fail to reproduce the actual dynamic behavior of the original amplifiers, and therefore, lack the desired “feel” and artistic satisfaction. Furthermore, for practical reasons, such schemes are typically incorporated in otherwise conventional solid-state amps, which still suffer from the afore-mentioned qualities of coldness and brittleness. Similarly, attempts to apply current and voltage feedback in varying proportions to a standard linear amplifier, thus providing a higher output impedance that enhances the sound of the speaker, may still result in clipping which is undesirably harsh and such amps have not been combined advantageously with other desirable processing as described above. The most common and commercially supported topologies present a low impedance voltage source to the speaker with the tonal drawbacks noted above.
Thus, there exists a desire for an amplifier and method with improved harmonic response and which overcomes one or more of the aforementioned drawbacks.